The present invention relates to an ion source for generating ion beams by d.c. discharge.
Kaufman-type and bucket-type ion sources are heretofore known as typical examples of ion sources used for ion beam sputtering, ion beam mixing, ion assisting, etc.
FIG. 1 is a block diagram of a Kaufman-type ion source which comprises a nonmagnetic metal cubic body 1 of stainless steel or the like, a plasma generating chamber 2 formed with the cubic body 1, a filament 3 provided in the plasma generating chamber 2, the filament 3 being composed of tungsten W, lanthanum boride (LaB.sub.6) or the like, a filament power source 4 for heating the filament 3, a tubular anode 5 provided in the plasma generating chamber 2, an inlet 6 for introducing gas into the plasma generating chamber 2, a group of ion-beam drawing electrodes 7 consisting of first electrodes 8, second electrodes 9 and third electrodes 10, a permanent magnet or electromagnet 11 provided outside the cubic body 1 and used for generating a cylindrical magnetic field, an arc source 12 whose positive electrode is connected to the anode 5 and whose negative electrode is connected via a resistor 13 to the cubic body 1, the arc source 12 being used for applying anode voltage to the anode 5, an accelerating power source 14 whose positive electrode is connected to the negative electrode of the arc source 12 so as to apply a positive accelerating voltage lower than the anode voltage to the first electrodes 8 which is at the same potential as that of the cubic body 1, and a decelerating power source 15 for applying a negative voltage to the second electrodes 9.
The filament 3 is a cathode which functions as a thermion emission source and is caused to have high temperatures by resistance heating derived from the filament power source to emit thermions e, In the case of tungsten or lanthanum boride, for instance, the filament is heated up to approximately 2,400.degree. C. or 1,300.degree. C.
On the other hand, ionizing gas, e.g., rare gas such as argon gas or reactive gas such as oxygen gas, is introduced from the gas inlet 6 into the plasma generating chamber 2.
A plasma 16 of ionized gas is generated by the discharge between the filament 3 and the anode 5 and, because of the beam drawing action of the group of electrodes 7, the ion gas in the plasma in the form of an ion beam is led out of cubic body 1 to a sputtering chamber.
The generation and enclosure of the plasma 16 is improved in efficiency due to the magnetic field of the magnet 11.
FIG. 2 shows the construction of a typical bucket-type ion source. The difference between this ion source and the one shown in FIG. 1 is as follows.
The bucket-type ion source shown in FIG. 2 is not equipped with an anode 5 and the anode of an arc power source 12 is directly connected to the cubic body 1. The cubic body 1 functions as the anode, and the anode of an accelerating power source 14 is connected directly or via a resistor to first electrodes 8 of the group of electrodes 7 with an insulator 17 positioned between electrodes 8 and body 1. Further, the permanent magnet 11 is composed of a plurality of annular bodies and disposed in such a manner as to form a cusp magnetic field. The ion source of FIG. 2 functions substantially similar to that of the ion source of FIG. 1.
A conventional hollow cathode type ion source is constructed as shown in FIG. 3. The difference between this ion source and that shown in FIG. 2 is as follows.
The ion source shown in FIG. 3 is not equipped with a filament 3 but has a cubic cathode body 19 made of non-magnetic metal fitted via an insulator 18 to the left-hand side opening of the cubic body 1, whereas a lid 21 made of non-magnetic metal is fitted via an insulator 20 to the left-hand side opening of the cubic body 19 so as to form a cathode chamber 22.
The cathode chamber 22 is provided with a hollow cathode 23, a heater coil 25 being wound on the outer periphery of a cylindrical body 24 incorporated in the lid 21. A thermion emitting material 26 is provided on the inner side of the cylindrical body 24 to form the hollow cathode 23. The heater coil 25 is heated by the power source 4 and the thermion emitting material 26 is heated via the cylindrical body 24.
The lid 21 is provided with a gas inlet 27 for guiding the gas into the cylindrical body 24.
The cathode of the arc power source 12 is connected to the lid 21, i.e., the cylindrical body 24 and also connected via a resistor 13 to the cubic cathode body 19 and the first electrodes 8.
Ionizing gas is introduced from the inlet 6 into the plasma generating chamber 2 and rare gas, such as argon for hollow-discharging, is introduced from an inlet 27 inside the cylindrical body 24. As the power source is activated, a discharge is caused between the cubic body 1 forming the anode and the hollow cathode 23. Rare gas hollow-discharging is generated in the inner space of the cylindrical body 24 and thermions e' are discharged from the thermion emitting material 26 because of the thermions e' thus emitted are introduced by the anode voltage into the generating chamber 2.
As a result, plasma 16 is generated in the plasma generating chamber 2 because of the discharge between the cubic body 1 and the hollow cathode 23. The discharge is made to continue by the thermions e, supplied from the cathode 23 and, as in the case of FIG. 2, the ionized gas from the gas inlet 6 is electrolytically dissociated, so that ion beams are led out via the group of electrodes.
The conventional ion source as shown in FIGS. 1 and 2 is designed to emit thermions e, required to generate plasma 16 by maintaining the filament 3 as an electron emitting source at high temperatures and simultaneously making use of the thermion emitting phenomenon. As a result, the material of the filament 3 heated to high temperatures evaporates and, while it is exposed to severe conditions such as sputtering derived from ion impacts, quickly wears. When the argon ionizing gas is employed, a tungsten filament and a lanthanum boride filament will last for approximately 50 and 100 hours, respectively. In addition to this problem, if a reactive ionizing gas, such as oxygen, is used the filament 3 will be quickly oxidized and therefore have a very short lifetime.
In the case of the ion source shown in FIG. 3, the hollow discharge resulting in thermion emission is created in the cathode chamber 22, which is different from the plasma generating chamber to which the ionizing gas is supplied. The ion source of the type shown in FIG. 3 can thus be used for longer periods than those shown in FIGS. 1 and 2. However, the thermion emitting material 26 will still be subjected to sputtering resulting from ion impacts at high temperatures even where the ionizing gas is a rare gas, such as argon, and the lifetime of the material 26 will be only for 100-200 hours.
In addition, since part of the ionizing gas introduced into the plasma generating chamber 2 is allowed to flow into the cathode chamber 22, the thermion emitting material 26 is consumed more quickly in case that the ionizing gas is an oxygen gas and the like rather than a rare gas.
Further, the hollow-discharging rare gas in the cylindrical body 24 is mixed with the ionizing gas in the plasma generating chamber 2. Consequently, the plasma 16 thus generated becomes a mixed one, comprising the ionizing gas and the hollow-discharging rare gas. Plasma generation relying on only the ionizing gas cannot be attained and the desired ion beams may become unavailable.